1. Field of the Invention
The present invention relates to a structure for and method of manufacturing a piezoelectric resonant part such as a chip type ceramic filter.
2. Description of Prior Art
Referring to FIG. 8, a prior art piezoelectric filter (which is one type of piezoelectric resonant part) includes a piezoelectric substrate 1 having predetermined electrode patterns (not shown) formed on its opposing main surfaces. The electrode patterns cooperate with the intervening piezoelectric substrate to form first and second vibrating elements which are spaced apart from one another. A pair of protective substrates 2, 3 are bonded to opposite main surfaces of the piezoelectric substrate 1 via adhesive layers 4, 5, respectively. External electrodes 6, 7, 8 are formed on the outer periphery of both the piezoelectric substrate 1 and the protective substrates 2, 3.
In a first known piezoelectric filter (hereinafter referred to a first prior art example), adhesive layers 4, 5 are made of a hard adhesive such as an epoxy resin whose Shore hardness is around D70 to D80 after hardening and the protective substrates 2, 3 are made of an epoxy material.
In a second known piezoelectric filter (hereinafter referred to as a second prior art example), the adhesive layers 4, 5 each have a one-layer structure composed of a soft epoxy resin whose Shore hardness is around D30 and the protective substrates 2, 3 are made of a magnesium titanate material.
While not shown in FIG. 8, it is common to provide vibration spaces for the vibrating elements of the electrode pattern by forming open spaces within the adhesive layers 4, 5 at positions corresponding to the vibrating elements in the piezoelectric filter having the structure of the first prior art example. In the piezoelectric filter having the structure of the second prior art example, however, concave portions for use as vibration spaces are formed on the internal surfaces of the protective substrates 2, 3 because it is difficult to assure adequate vibration spaces if openings are only formed in the soft adhesive layers 4, 5.
Piezoelectric filters of this type are often mounted on a circuit board using a reflow soldering process. When the piezoelectric filter having the structure of the first prior art example is mounted using this process, the filter sometimes exhibits a significant change of frequency. This is especially true when the piezoelectric filter is located in a humid atmosphere. It is believed that such problems are caused by stresses applied to the piezoelectric substrate 1 resulting from expansion or contraction of the glass/epoxy resin protective substrates 2, 3 due to the infiltration of moisture.
On the other hand, the piezoelectric filter having the structure of the second prior art example has drawbacks in that it takes time to prepare the protective substrates 2, 3 and it is costly to form the concave portions in the magnesium titanate protective substrates 2, 3 for use as the vibration spaces. Further, the thickness of the adhesive layers 4, 5 has to be kept thin in order not to disconnect the electrodes 6, 7, 8 because the adhesive layers 4, 5 are formed using a soft adhesive. However, the thin adhesive layers 4, 5 cannot exert a desired damping effect, thus causing the filter to exhibit significant spurious characteristics.
It is one object of the present invention to solve the aforementioned problems by providing piezoelectric resonant part, and a method for manufacturing such a part, which does not require concave portions to be formed on the internal surfaces of the protective substrates in order to provide the necessary vibration space for the vibrating elements and which can prevent the outer electrodes from being disconnected.
It is another object of the present invention to provide a piezoelectric filter which exhibits fewer spurious characteristics while suppressing changes of frequency due to the infiltration of moisture.